Dirigible landing.

I know that modern day blimps use balloonets (Sorry if I misspelled that) to land by adding air to the main envelope, thus displacing lighter than air helium with air. And they can also simply direct the force of the propellers upward (This includes elevator flaps). And you can always vent helium, but that is an incredible waste of money.

Other than these ways I listed, what are some other ways that dirigibles could land while remaining independent of the ground?

You can have a pump that takes helium from the balloon and compresses it into a bottle. Pity: without the pump you don't land.

I made in 1982 a balloon with helium and a separated bit of ammonia (please choose a non-toxic gas) that I pumped from an ammonia solution. More gas volume from the same pressure. One advantage more: I reduced the bottle pressure to <1bar with the pump, so a pump failure would land the balloon.

Maybe adsorption can enable a non-toxic gas. It can be heavier than air.

Pull a piston in a cylindre to have a lower pressure inside, possibly vacuum? More difficult to build light.

Evaporate a liquid or solid with heat? It looks worse than heating air or pumping the vapour.

The standard method combines a helium balloon with a hot air balloon. Saves heating power.

Heat the helium?

The present designs I know rely solely on propellers. Sky cranes would have transported a ballast to compensate the load.

Would it be possible to use the equivalent of a fish's swim bladder? This is a small bladder of gas in the abdomen which is squeezed by muscles. It is possible to make some extremely light and strong envelopes these days which could be filled with Helium and wrapped around a cylinder to reduce the volume when you need to land.

- In theory, yes.
- In technology, feasible. You might for instance fill with gas a long tube with thin flexible walls, have a helical reinforcement on the walls like aramide fibre, and twist the whole tube with an electric motor so that the reinforcement squeezes the tube.

But it's imperfect:
- If losing power you can't land any more! It would at least need some sort of huge spring to guarantee passive safety.
- Compressing a gas from 1atm needs much energy. Say, to reduce the lift by just 2kN (200kg) you must bring 1000m3 from 1atm to 1.2atm at constant temperature which takes 20MJ or 12V*460Ah, oops! 10 car batteries emptied.

Compare to competitor solutions:
- I prefer to pull an under-pressure or vacuum (for passive safety) even if walls are heavier.
- With my ammonia dissolved in water, volume can change a lot with little pressure difference. Or try to adsorb. Or find a chemical reaction like NO2 <=> N2O4 but less toxic.
- You could evaporate or condense a fluid. Very similar to dissolution, but the altitude is unstable (like in a thundercloud) and temperature change has unwanted effects.
- Or you get much heat from little fuel and convert the heat in gas volume, which isn't inefficient. Bring initial 1000m3 of helium from 300K to 360K as previously, it takes 56MJ obtained from 1.3kg of kerosene.

Sit down comfortably, relax, and admire, because I've found ZE solution.
Well, maybe it exists already, but as usual I didn't check, so interested people should.

I propose to control the aerostat's buoyancy through the temperature of a gas, but with little heat expense and a short reaction time, by using a heat storer-exchanger about as a Stirling engine does.

Such a storer-exchanger has a a warm and a cold side and lets the fluid flow from cold to warm to warm it, and in the reverse direction to cool it.
- Heat removed from the fluid gets back in the storer-exchanger for re-use;
- The fluid has everywhere nearly the temperature of the storer-exchanger, avoiding losses;
- The same fluid gives and receives heat at different times. No need to withstand pressure nor transmit heat through a wall. The exchange surface is easily made huge.

By moving a gas through a storer-exchanger in one direction or the other, its temperature and volume hence buoyancy is controlled quickly and reversibly: very nice for the aerostat. Heat is lost only through technological imperfections or if the aerostat delivers cargo higher than it picked it.

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The gas can be helium (needs less heat for being monoatomic), air, or an other, even heavier than air.

The insulating envelope, the storer-exchanger... can be outside or within the aerostat's main hull, and might even be the main hull. If within the main hull, heat leaks through the insulating envelope don't reduce immediately the total buoyancy - but they affect the buoyancy's control range.

On the sketch, the storer-exchanger itself moves in a stiff insulating envelope, but this envelope could be deformable, or a separate piston could move the gas - combine several methods if you like worries.

Smooth operation, including stability, wants to keep the warm gas at nearly the warmer temperature of the storer-exchanger. The gas can be blown, or rely on natural convection, for instance if it's above the storer-exchanger.

A very intimate contact between the gas and the storer-exchanger is compatible with a small pressure drop. It needs finely divided gas channels, suggesting a clean gas. Many narrow and short capillaries, fed in parallel by arteries, arterioles, veinules and veins like for blood, possibly in different directions, make a superior organisation. One example is a porous ceramic with in and out channels on a chessboard pattern, but stapled sheet would achieve it too, as would loose powder, or a metal or ceramic mesh, or thin nickel that separates the gas from a heat storing liquid - and more possibilities.

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For the storer-exchanger, water possibly mixed with antifreeze, brine (of LiF better than NaCl?), and other materials like liquid or solid paraffins (polyolefins) have a big heat capacity but a limited temperature range - as may the insulating envelope. A liquid, or solid-liquid transitions preferably at successive temperatures, should be immobilized to preserve a warm and a cold side.

Take again a 2kN (200kg) buoyancy control. 625m3 of helium getting 80K would need ~500kg of brine to lose 20K.

A bigger relative temperature variation in the storer-exchanger is acceptable if:
- Gas exiting it doesn't mix, so gas' temperature corresponds to the storer-exchanger when it enters again;
- Sections of the storer-exchanger are switched so its extreme temperatures correspond to the gas at both sides;
or use solid-liquid transitions, with proper regulation.

Metal, ceramic... enable a hotter storer-exchanger but offer only some 800W/kg/K. Li, Be, B bring more but have drawbacks.

The storer-exchanger can be heated before flight, possibly outside the main envelope, by a combustion, electricity, Sunlight... and be further heated or not during flight.

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A stiff insulating envelope can consist of foam, or a possibly multilayer honeycomb with organic spacer... They can be filled with a better insulator than helium, like argon or vacuum. Aramide, polyimide have good heat resistance - and a ceramic foam would be welcome.

If flexible, the insulating envelope might use a foam as well, or mesh between several films... Roll it like a toothpaste tube?

If surrounding the warmer gas mainly by the storer-exchanger, for instance as two big pistons in a short cylinder, then leaks decrease.

The reaction time is better! <1s looks feasible with the exchanger (Stirling engines rotate faster than 60rpm) which enables to control an airship through its buoyancy and not only by its propellers or ballast. If used as a crane, say to install a wind turbine, this changes everything.

The mass looks less favourable. The same 2kN lift costs once 500kg of heat accumulator instead of 1kg of kerosene or propane each time.

Melting-freezing may save mass. Alloys like Galinstan exist for low temperatures; stages with varied compositions would bring the small temperature steps.

Polyols and salts resist fire better; the best ones can weigh slightly less than paraffins. Alloys are heavier.

The accumulator-exchanger must have stages of successive temperatures. To hold the liquid, one might encapsulate the material when solid in a thin layer of catalytic nickel, with a bubble or foam inside to allow for expansion. Many pebbles, wires... would give the heat exchange area. A classical liquid/gas heat exchanger fits as well.

An airship that can adjust often and quickly its buoyancy would be good for sightseeing tours.

Presently, they adjust the ballast to the passengers, a rather lengthy operation needing people on the ground. These airship are rather big and carry a dozen of tourists for hours.

An alternative is a turbine helicopter that takes two passengers on a shorter tour, but it's pretty expensive.

Imagine the operation of a small airship with my regenerative buoyancy control and maybe silent electric propulsion: take a pair of tourists, make a turn around the Pão de Açúcar (or the Iguazu waterfalls, the Taj Mahal, Angkor Wat... There are many candidates), land after half an hour, take the next customers.